aarch64: Fix .cfi_window_save with pac-ret [PR94515]

aarch64: Fix .cfi_window_save with pac-ret [PR94515]

On aarch64 -mbranch-protection=pac-ret reuses the dwarf
opcode for window_save to mean "toggle the return address
mangle state", but in the dwarf2cfi internal logic the
state was not updated when an opcode was emitted, the
currently present update logic is only valid for the
original sparc use of window_save so a separate bool is
used on aarch64 to track the state.

This bug can cause the unwinder not to authenticate return
addresses that were signed (or vice versa) which means a
runtime crash on a pauth enabled system.

Currently only aarch64 pac-ret uses REG_CFA_TOGGLE_RA_MANGLE.

This should be backported to gcc-9 and gcc-8 branches.

gcc/ChangeLog:

	PR target/94515
	* dwarf2cfi.c (struct GTY): Add ra_mangled.
	(cfi_row_equal_p): Check ra_mangled.
	(dwarf2out_frame_debug_cfa_window_save): Remove the argument,
	this only handles the sparc logic now.
	(dwarf2out_frame_debug_cfa_toggle_ra_mangle): New function for
	the aarch64 specific logic.
	(dwarf2out_frame_debug): Update to use the new subroutines.
	(change_cfi_row): Check ra_mangled.

gcc/testsuite/ChangeLog:

	PR target/94515
	* g++.target/aarch64/pr94515-1.C: New test.
	* g++.target/aarch64/pr94515-2.C: New test.